/*main.h*/
#include <foo.h>
#include <faa.h>

bool can_use_foo = false;
bool can_use_faa = false;

#ifdef _FOO_INCLUDED_
can_use_foo = true;
#endif

#ifdef _FAA_INCLUDED_
can_use_faa = true;
#endif

void useFooDoSomething( void )
{

     if( can_use_foo )
     {

          fooDoSomething();

     }
     else
     {

          cout << "Foo was not found. Cannot use Foo's fooDoSomething() function.\n";

     }

}

void useFaaDoSomething( void )
{

     if( can_use_faa )
     {

          faaDoSomethingDifferent();

     }
     else
     {

          cout << "Faa was not found. Cannot use Faa's faaDoSomethingDifferent() function.\n";

     }

}

/*main.cpp*/
#include "main.h"

int main()
{

     useFooDoSomething();
     useFaaDoSomething();
     return 0;

}